Obfer vat ions on EletJricity . 269 
but that when the excitation is weak, on account of the eleclric 
matter not paffing with fufficient facility to the rubbed furface, 
the friflion enables the oppofite furface to attradl or receive it, and 
if it be fupplied, both furfaces will pafs off in the pofitive Rate ; 
and either furface will give out more eleflricity than is really in- 
duced upon it, becaufe the eleflricity of the oppofite furface forms 
a charge. It may be neceflary to obferve, that I am fpeaking of 
the fails or effeils produced by fri&ion ; but how the rubbing 
furfaces ail upon each other to produce them, whether by 
attrailion, or otherwife, I do not here enquire. 
It will hereafter be feen, that plate machines do not colleil 
more eleilricity than cylinders (in the hands of the eleilrical 
operators of this metropolis) do with half the rubbed furface ; 
which is a corroboration of the inference here made. 
16. When a cylinder is weakly excited, the appearances 
mentioned (par. 8.) are more evident, the more rapid the turn- 
ing. In this cafe, the avidity of the furface of the cylinder 
beneath the filk is partly fupplied from the edge of the filk 
which throws back a broad cafcade of fire, fometimes to the 
diflance of above twelve inches. From thefe caufes it is that 
there is a determinate velocity of turning required to produce the 
maximum of intenfity in the conduflor. The ftronger the ex® 
citation the quicker may be the velocity ; but it rarely exceeds 
five feet of the glafs to pafs the cufhion in a fecond. 
17. If a piece of filk be applied to a cylinder, by drawing 
down the ends, fo that it may touch half the circumference, 
and the cylinder be then turned and excited by applying the 
amalgamed leather, it will become very greedy of ele&ricity 
during the time it pafifes under the filk. And if the entering 
furface of the glafs be fupplied with eleclncity, it will give it 
out at the other extremity of contafl ; that is to fay, if infu- 
Vol. LXXIX, R r lated 
